With the gradual depletion of hydrocarbon reserves found offshore, there has been considerable attention attracted to the drilling and production of oil and gas wells located in water. In relatively shallow water, wells may be drilled in the ocean floor from bottom founded, fixed platforms. Because of the large size of the structure required to support drilling and production facilities in deeper and deeper water, bottom founded structures are limited to water depths of less than about 1,000-1,200 feet. In deeper water, floating drilling and production systems have been used in order to reduce the size, weight, and cost of deep water drilling in production structures. Ship-shape drill ships and semi-submersible buoyant platforms are commonly used for such floating facilities.
When a floating facility is chosen for deep water use, motions of the vessel must be considered and, if possible, constrained or compensated for in order to provide a stable structure from which to carry on drilling and production operations. Rotational vessel motions of pitch, roll and yaw involve various rotational movements of the vessel around a particular vessel axis passing through the center of gravity. Thus, yaw motions result from a rotation of the vessel around a vertically oriented axis passing through the center of gravity. In a similar manner, for ship-shape vessels, roll results from rotation of the vessel around the longitudinal (fore and aft) axis passing through the center of gravity causing a side to side roll of the vessel and pitch results from rotation of the vessel around a lateral (side to side) axis passing through the center of gravity causing the bow and stern to move alternatively up and down. With a symmetrical or substantially symmetrical platform such as a common semi-submersible, the horizontally oriented pitch and roll axes are essentially arbitrary and, for the purposes of this disclosure, such rotations about horizontal axes will be referred to as pitch/roll motions.
All of the above vessel motions are considered only relative to the center of gravity of the vessel itself. In addition, translational platform motions must be considered which result in displacement of the entire vessel relative to a fixed point, such as a subsea wellhead. These motions are heave, surge and sway. Heave motions involve vertical translation of the vessel up and down relative to the floatably fixed point along a vertically oriented axis passing through the center of gravity. For ship-shape vessels, surge motions involve horizontal translation of the vessel along a fore and aft oriented axis passing through the center of gravity. In a similar manner, sway motions involve the lateral, horizontal translation of the vessel along a left to right axis passing through the center of gravity. As with the horizontal rotational platform motions discussed above, the horizontal translational motions, surge and sway, in a symmetrical or substantially symmetrical vessel such as semi-submersible are essentially arbitrary and, in the context of this specification, all horizontal translational vessel motions will be referred to as surge/sway motions.
Combinations of the above-described motions encompass platform behavior as a rigid body in six degrees of freedom. The six components of motion result as responses to continually varying harmonic wave forces. These wave forces are first said to vary at the dominant frequencies of the wave train. Vessel responses in the six modes of freedom at frequencies corresponding to the primary periods characterizing the wave trains are termed "first order" motions. In addition, a variable wave train generates forces on the vessel at frequencies resulting from sums and differences of the primary wave frequencies. These are secondary forces and corresponding vessel responses are called "second order" motions.
A completely rigid structure fixed to the sea floor is completely restrained against response to the wave forces. An elastic structure, that is elastically attached to the sea floor, will exhibit degrees of response that very according to the stiffness of the structure itself, and according to the stiffness of its attachment to the earth at the sea floor. A "compliant" offshore structure is usually referred to as a structure that has low stiffness relative to one or more of the response modes that can be excited by first or second order wave forces.
Floating production or drilling vessels have essentially unrestricted response to first order wave forces. However, to maintain a relatively steady proximity to a point on the sea floor, they are compliantly restrained against large horizontal excursions by a passive spread catenary anchor mooring system or by an active controlled-thruster dynamic positioning system. These positioning systems can also be used to prevent large, low frequency (i.e. second order) yawing responses.
While both ship-shaped vessels and conventional semi-submersibles are allowed to freely respond to first order wave forces, they do exhibit very different response characteristics. The semi-submersible designer is able to achieve considerably reduced motion response by: (1) properly distributing buoyant hull volume between columns and deeply submerged pontoon structures, (2) optimally arranging and separating surface-piercing stability columns and (3) properly distributing platform mass. Proven principles for these design tasks allow the designer to achieve a high degree of wave force cancellation such that motions can be effectively reduced over selected frequency ranges.
The design practices for optimizing semi-submersible dynamic performance depend primarily on "detuning" and wave force cancellation to limit heave. Pitch/roll responses are kept to acceptable levels by providing large separation distances between the corner stability columns while maintaining relatively long natural periods for the pitch/roll modes. This practice keeps the pitch/roll modal frequencies well away from the frequencies of first order wave excitation and is, thus, referred to as "detuning". Wave force cancellation is achieved by properly distributing submerged volumes comprising the hull relative to the elements that penetrate water surface.
Another class of compliant floating structure is moored by a vertical tension leg mooring system. The tension leg mooring also provides compliant restraint of the second order horizontal motions. In addition, such a structure stiffly restrains vertical first and second order responses, heave and pitch/roll. This form of mooring restraint would be essentially impossible to apply to a conventional ship-shape monohull due to the wave force distribution and resultant response characteristics. Therefore, this vertical tension leg mooring system is generally conceived to apply to semi-submersible hull forms which can mitigate total resultant wave forces and responses to levels that can be effectively and safely constrained by stiffly elastic tension legs.
This type of floating facility, which has gained considerable attention recently, is the so-called tension leg platform (TLP). The vertical tension legs are located at or within the corner columns of the semi-submersible platform structure. The tension legs are maintained in tension at all times by insuring that the buoyancy of the TLP exceeds its operating weight under all environmental conditions. When the buoyant force of the water displaced by the platform/structure at a given draft exceeds the weight of the platform/structure (and all its internal contents), there is a resultant "excess buoyant force" that is carried as the vertical component of tensions in the mooring elements (and risers). When stiffly elastic continuous tension leg elements called tendons are attached between a rigid sea floor foundation and the corners of the floating hull, they effectively restrain vertical motions due to both heave and pitch/roll inducing forces while there is compliant restraint of movements in the horizontal plane (surge/sway and yaw). Thus, a tension leg platform provides a very stable floating offshore structure for supporting equipment and carrying out functions related to oil production. Conoco's Hutton platform in the North Sea is the first commercial example of a TLP. Saga's Snorre platform, being constructed for the North Sea, is a later example of a TLP.
The primary interest in the TLP concept is that the stiff restraint of vertical motions makes it possible to tie-back wells drilled into the sea floor to production facilities on the surface through a collection of pressure containment apparatuses (e.g., the valves of a well "tree") such that the "tree" is located above the body of water within the dry confines of the platform's well bay. This "dry tree" concept is very attractive for oil field development because it allows direct access to the wells for maintenance and workover. As water depth (and, thus tendon length) increases, tendons of a given material and cross-section become less stiff and less effective for restraining vertical motions. To maintain acceptable stiffness, the cross-sectional area must be increased in proportion to increasing water depth. For installations in deeper and deeper water, a tension leg platform must become larger and more complex in order to support a plurality of extremely long and increasingly heavy tension legs and/or the tension legs themselves must incorporate some type of buoyancy to reduce their weight relative to the floating structure. Such considerations add significantly to the cost of a deep water TLP installation. Conoco's Jolliet TLWP (Tension Leg Well Platform) in the Gulf of Mexico addresses this problem by citing production equipment on a nearby conventional platform in shallower water. However, this approach is limited to locations that have sites relatively nearby for the production equipment.
In addition, in deeper and deeper water, a greater percentage of the hull displacement must be dedicated to excess buoyancy (i.e. tendon pretension) to restrict horizontal offset. Station-keeping is a key role for the mooring system. The vertical tension leg mooring system provides the capacity to hold position above a fixed point on the sea floor as any horizontal offset of the platform creates a horizontal restoring force component in the angular deflection of the tendon tension vector. In deeper and deeper water, it requires greater tendon pretension to provide enough restoring force to keep the TLP within acceptable offset limits. This increases leads to larger and larger minimum hull displacements. As in aircraft and motor vehicle design, there is a multiplying effect. That is, each unit of additional weight requires additional structural weight to support it which in turn requires still more weight or mass of the structure. Thus, any decrease in weight or mass of essential elements leads to considerable savings.
This art was further advanced, in respect to limiting the impact of increasing water depth on the size, cost, and complexity of the mooring system and platform, with the disclosure of a single leg tension platform (STLP) in U.S. Pat. No. 4,793,738. In accordance with that invention, a single leg tension platform (STLP) was disclosed to comprise a large central buoyant column surrounded by a number of peripheral stability columns. In a preferred embodiment, peripheral stability columns were disclosed to be symmetrically spaced about the central column. The central column and the peripheral stability columns were disclosed to be connected together as one structure, the connection in one embodiment taking the form of an arrangement of subsea pontoons which rigidly connect the various columns near their lower ends and/or key structural bracing penetrating the water surface. The columns, especially the central column, support a deck from which drilling and other operations can be conducted.
Further in accordance with that invention, the STLP has a mooring system which incorporates both a vertical single tension leg system and a lateral (e.g., spread catenary) mooring system. The vertical tension leg is arranged so that it effectively restrains only the heave component of the vertical motions. The vertical tension leg mooring system and the spread mooring are disclosed to act in concert to compliantly restrain low frequency horizontal motions, surge/sway and yaw. The use of a hybrid mooring system as described for that invention reduces the impact of increasing water depth on minimum hull displacement and tendon pretension and thus reduces weight and cost.
There continues to be a compelling need for improved platforms and drilling systems, particularly those which are less costly and safer, for production of hydrocarbons from beneath relatively deep water, particularly water depths of 500 feet to 8000 feet, and more particularly 1000 to 4000 feet. Unless this need is satisfied, only very rich reservoirs will support development at such relatively great depths. Therefore, it is appropriate to examine all aspects of deep water drilling and production systems in order to identify those features which are most sensitive to increasing water depths. In this regard, it is necessary to give careful consideration to both drilling and well systems, and tie-back riser design.
As water depth increases, the risers become naturally longer just as the tendons do, as discussed above. To achieve proper top end support so as to limit riser responses in severe metocean conditions, riser top tensions must be increased at a greater rate than the rate by which water depth is increased. Therefore, risers and riser tensions tend to place an ever increasing load on the floating (TLP) structures as they are placed in deeper waters.
Further as offshore development moves to deeper waters, the drilling environment can change in a manner such that any wells being drilled through the various subterranean formations will encounter "over-pressured" zones where fluids are charged with a formation pressure which exceeds the pressure head that can be supplied by a correspondingly deep (or high) column of water. These well "over-pressures" are normally contained/controlled by a multiplicity of pressure containment means. It is considered standard practice that at least two of these pressure containment means be independent of each other. In deep water, situations can occur where the pressure containment provided by a special well control fluid (a mixture denser than water that is usually called "mud") and the pressure containment provided by a tie-back casing/riser+surface "tree" are not independent. In these situations (which are commonplace for deep water wells in the Gulf of Mexico for example), a leak in the casing/riser near the seabed could result in loss of so much well control fluid from riser that the formation pressure down-hole would not be contained. The result would be a "blow-out". In order to ensure that a leak in the primary casing does not result in complete loss of well control, it has been practiced that a second casing string has been employed surrounding the primary pressure containing casing (e.g., a concentric casing riser design to be employed on the Shell "Auger" platform). Such a measure is a reasonable practice, but it does result in a much heavier riser string to be supported by top tension at the floating platform. The increased riser tensions lead to much larger platform dimensions and cost.